fiberglass modification for new shower faucet in old shower

I have a cheap fiberglass 1-piece tub/shower surround... about 15 yrs old. still has the cheap builder-grade faucet. I got a great deal on a new faucet and removed the old faucet. I realized that the hole was drilled in the fiberglass prior to install of the surround over the old faucet and the surround hole is smaller than the faucet casing. I have no access to the old faucent and plumbing because of the limited size of the hole in the fiberglass. SO, I have no center hole to use for drilling. How can I cut out the hole to be larger without damaging the wall or pipes? I kind of need to eliminate as much risk as possible so I don't damage the surround and turn this into a big project. I have a jigsaw. Can I just be careful to not cut blindly where the threaded pipe ends are (and soon will be after installed) on the new faucet? that would help me guess where the pipes are and not damage them. Are they all pretty standard pipe locations for tub/shower faucets if it was all installed up to code by the builder?

If your old faucet has threaded ends and the new one does too, there is little likelihood that you will be able to change it through the opening in the fiberglass surround. That type of installation usually requires a lot more room than that.